The only reason why I can't sometimes kill for no reason is because of empathy, I don't want to spoil mood of someone else, if I don't know this person, that's why I usually trying to see the worst in people and always find a reason for bad actions, or else I would feel myself bad, if actions wasn't lawful and justified. Even when on troll alt I convienced newbie to bring me to his tent because "i can upgrade it", I couldn't rob him, he was so naive, in his bluesuit, standing and waiting till I start upgrading his tent... I said that I'm not tent-upgrader and that it's impossible to do that in game, I just a "tent-robber", I checked his box, there were alot items, my troll alt is unarmed character, I asked him if I can borrow mega-powerfist from his box, he wasn't against it... everything else I left in box and simply left his tent. Even when I was abusing idolized stuff and merc "oops"ing I didn't killed anyone for no reason, with idolized I was shooting not-so-good people who PKing around NCR and those who shot me in NCR 1st so I simply had to self-defence, with merc abuse I only brought some guys to negative HP and then stoped shooting, everyone who died by this abuse did it because of thier own fault by guards. The only time when I killed innocent was bombing with dynamite, and even there I tried to bomb blues who has nothing to lose... well, actually I didn't know that because I couldn't check if they had items or not, and often they had, because thier bodies whispered bad words about me and about thier lost items/caps.
So, personally, my opinion, if you see someone PKing you and they did it not because you were a threat for them, but because they like to kill people, it's thier problems, you see that they have problems and it's enough for you, everyone has thier own life, just leave them with it, may be someone has bad life where they're victims for no reason, so they're trying to be hunters in FOnline.